1. Field of the Invention
The invention relates to superconductive circuits particularly with regard to a novel Josephson tunnel junction device therefor.
2. Description of the Prior Art
Superconductive Josephson memory and logic circuits are known in the art which utilize the Josephson tunnel junction as the active switching elements or gates therefor. The Josephson junction comprises two superposed layers of superconductive material with an insulator or semiconductor layer or barrier therebetween whereby Josephson tunneling current flows from one superconductive layer to the other through the barrier via the Josephson tunneling effect. With the superconductive layers connected into a superconductive loop and control lines disposed adjacent the junction the d.c. Josephson zero voltage current flowing through the device may be controlled so as to provide the necessary current steering control functions in the Josephson circuitry.
Primary objectives in the design of Josephson circuitry are to minimize the surface area required for the circuit components such as the memory and logic superconductive loops and to provide fabrication parameters that result in relatively simple and non-critical manufacturing procedures that reliably produce high yield circuits.
One of the pertinent parameters in designing Josephson storage loops of minimum area is the loop inductance. The value of inductance is a direct function of loop size. The smaller the loop the lower will be its inductance value. Another factor influencing loop size is the quantum mechanical effect which is an inherent property of superconductive circuits. Magnetic flux stored in a loop is necessarily quantized in magnetic flux units determined by Planck's constant and the charge of the electron. Only integral numbers of flux quanta can be stored in a loop. Thus the circulating loop supercurrent related to the loop flux is necessarily limited to quantized values thereof.
The magnetic flux threading a superconductive loop is equal to the product of the circulating loop supercurrent and the loop inductance. Since the minimum amount of flux that can be stored in a superconductive loop is one flux quantum, the minimum circulating supercurrent that can flow in a loop of given inductance is the quantized current value corresponding thereto. It is generally desirable for reliable switching to store a plurality of flux quanta with the corresponding quantized supercurrent circulating in the loop although circuits may be utilized that store one flux quantum. It is appreciated, therefore, that the smaller the loop size, and hence the smaller the inductance value, the greater must be the value of the circulating supercurrent to provide storage of reasonable numbers of flux quanta.
The supercurrent circulating in the loop passes through and must be supported by the Josephson tunnel junction switches utilized therein. For reasonable barrier thicknesses and conventional deposited barrier materials, present day Josephson tunnel junction switches provide limited current densities of between tens of milliamperes per square centimeter to several hundred amperes per square centimeter. With such current densities, in order to support the required circulating supercurrents in loops of reasonable dimensions, unduly large junction areas are required to the extent that the size of area required for the junction may approach the total size allocated for the loop with its tunnel junction switches, otherwise extremely thin oxide barriers must be used to support high critical current densities.
Although critical current density increases with decreasing thickness of the barrier, thin barriers are undesirable since they tend to be fragile thus complicating fabrication procedures as well as tending to provide non-reproducible junction properties. With such fragile barriers only gentle deposition techniques may be utilized such as evaporative deposition for forming the upper superconductive layer in the sandwiched construction of the junction devices. With such thin layers, sputter deposition tends to be too violent a process, often consuming the thin layers or causing short circuits therethrough. However, when utilizing evaporative deposition, in order to obtain high superconductive transition temperatures for desirable superconductive materials, ultra high vacuum systems are required which tend to be expensive and critical in operation. For example, although niobium layers having a high transition temperature may be deposited by sputtering, at a background partial pressure of approximately 10.sup.-7 Torr of oxygen, evaporative deposition of the material to obtain a high transition temperature requires a background pressure of approximately 10.sup.-10 Torr.
The prior art has contemplated utilizing semiconductor materials as the barrier in Josephson tunnel junctions. It is expected that reasonably thin single crystal silicon may be utilized to achieve high current densities. In order to use such a barrier, complex thinning processes are required to reduce the thickness of the bulk silicon material to thicknesses usable in tunneling barriers. Once such thinning is achieved, patterning and deposition are required on both sides of the silicon wafer in order to provide the tunnel junction structure of superconductor-semi-conductor-superconductor. Thus fabrication techniques necessarily more complex than those utilized for one sided lithographic patterning and deposition are required.
In order to achieve optimum circuit performance it is desirable that the Josephson circuit loops, including the tunnel junctions therefor, form critically damped circuits. In order that a loop be critically damped it is required that .sqroot.L/C is greater than a critical resistance, where L is primarily the loop inductance and C is primarily the capacitance of the Josephson tunnel junction. As discussed above, it is desirable to maintain a small loop area to minimize the size of the Josephson circuits and, as discussed, the requirement of small loop size limits the magnitude of the loop inductance. Thus in order to achieve critical damping with small sized loops it is necessary to minimize the device capacitance. On the other hand, as discussed above, it is generally required to utilize relatively thin barriers to achieve the necessary supercurrent for the loop operation. This has a tendency to increase the device capacitance since capacitance varies inversely as the thickness of the barrier. Thus the prior art Josephson junction devices suffer from the disadvantage that a compromise must be effected between barrier thickness and available supercurrent, which compromise may often only be effected by the use of a relatively large area junction. Additionally the device capacitance varies directly with the dielectric constant of the barrier material. The dielectric constant of many prior art barrier materials are unduly high, therefore resulting in undesirably large device capacitance. For example, niobium oxide (Nb.sub.2 O.sub.5) has a dielectric constant of 30 which results in an undesirably large capacitance per unit area of barrier.